The present invention relates to strings for use in rackets for ball games, for example, tennis, squash and badminton, which are hollowed and filled with gelatinous oil.
In conventional strings, as substitutes for natural strings made from sheep's intestines, there are widely used solid strings made of synthetic resins. These strings, however, are inferior to hollow strings in strength. Such defect may be ascribable to the fact that, in the manufacture of the synthetic resin monofilament forming a core of strings by molding, in case of elongating the core having desired diameter, a big difference between the central elongation distance (a)-(a') and the outer elongation distance (b)-(b'), as shown in FIG. 4, causes the difference in strain between the molecules of the resins. In this case, the thicker the core is, the bigger the difference in strain becomes, which causes lack of uniformity in the elongation of the core. In fact, for example, in the polyamide fiber used for cores, the strength of the thicker one is inferior to that of the thinner one. Compared with the solid core, in the hollow core, as shown in FIG. 3, non-uniform elongation can hardly be seen because only the circumferential portion of the core may be elongated independent of the central portion thereof, which is hollowed.
Each of solid strings and hollow strings are made by helically winding a wrapping thread about a solid core and a hollow core respectively. Thus, when each of the solid strings and the hollow strings are spread out on rackets with tension of 60-75 lb, a big difference becomes apparent in strength, durability and ball-striking touch.
Spreading of strings on rackets is usually effected by using a spreading machine under a fixed tension. In this case, when the hollow strings are spread as such, air is charged into the inner portion of the strings, which causes compression strain in the strings on striking balls resulting in lowering of bouncing force of the rackets.
For eliminating these drawbacks, there had been developed hollow strings into the hollowed inside of which liquid oil is charged (cf. Japanese Utility Model Registration No. 1106964). However, since this liquid oil has low viscosity and fluidity at normal temperature, for preventing the oil from flowing out, the string's end is required to be sealed with a stopper. In addition, when each string is cut in a desired length, upon spreading the strings on rackets, not only the oil flows out from the hollow of the strings to contaminate clothes or the like, but also air enters into said hollow of the strings to lower the bouncing force as described above.